Activation of the Wnt/ß-catenin pathway in pancreatic beta cells during the compensatory islet hyperplasia in prediabetic mice.

The Wnt/ß-catenin signaling pathway, also known as the canonical Wnt pathway, plays a role in cell proliferation and differentiation in several tissues/organs. It has been recently described in humans a relationship between type 2 diabetes (T2DM) and mutation in the gene encoding the transcription factor TCF7L2 associated to the Wnt/ß-catenin pathway. In the present study, we demonstrated that hyperplastic pancreatic islets from prediabetic mice fed a high-fat diet (HFD) for 60 d displayed nuclear translocation of active ß-catenin associated with significant increases in protein content and gene expression of ß-catenin as well as of cyclins D1, D2 and c-Myc (target genes of the Wnt pathway) but not of Tcf7l2 (the transcription factor). Meanwhile, these alterations were not observed in pancreatic islets from 30 d HFD-fed mice, that do not display significant beta cell hyperplasia. These data suggest that the Wnt/ß-catenin pathway is activated in pancreatic islets during prediabetes and may play a role in the induction of the compensatory beta cell hyperplasia observed at early phase of T2DM.

Reduced nuclear protein 1 expression improves insulin sensitivity and protects against diet-induced glucose intolerance through up-regulation of heat shock protein 70.

We recently reported that deletion of the stress-regulated nuclear protein 1 (Nupr1) protected against obesity-associated metabolic alterations due to increased beta cell mass, but complete Nupr1 ablation was not advantageous since it led to insulin resistance on a normal diet. The current study used Nupr1 haplodeficient mice to investigate whether a partial reduction in Nupr1 expression conferred beneficial effects on glucose homeostasis. Islet number, morphology and area, assessed by immunofluorescence and morphometric analyses, were not altered in Nupr1 haplodeficient mice under normal diet conditions and nor was beta cell BrdU incorporation. Glucose and insulin tolerance tests indicated that there were no significant changes in in vivo insulin secretion and glucose clearance in Nupr1 haplodeficient mice, and beta cell function in vitro was normal. However, reduced Nupr1 expression decreased visceral fat deposition and significantly increased insulin sensitivity in vivo. In contrast to wild type animals, high fat diet-fed Nupr1 haplodeficient mice were not hyperinsulinaemic or glucose intolerant, and their sustained insulin sensitivity was demonstrated by appropriate insulin-induced Akt phosphorylation, as determined by Western blotting. At the molecular level, measurements of gene expression levels and promoter activities identified Nupr1-dependent inhibition of heat shock factor-1-induced heat shock protein 70 (Hsp70) expression as a mechanism through which Nupr1 regulates insulin sensitivity. We have shown for the first time that Nupr1 plays a central role in inhibiting Hsp70 expression in tissues regulating glucose homeostasis, and reductions in Nupr1 expression could be used to protect against the metabolic defects associated with obesity-induced insulin resistance.